In semiconductor laser devices, structures for limiting a current in a striped shape have been widely used for the purpose of decreasing an operating current and limiting the position of a light emitting spot. One of the structures for limiting a current in a striped shape is a structure having a current blocking layer for cutting off a current in a region, other than an opening, provided in a striped shape.
FIG. 22 is a schematic sectional view showing an example of the construction of a conventional GaN based semiconductor laser device having a current blocking layer.
In a semiconductor laser device 101 shown in FIG. 22, an n-contact layer 103 composed of n-GaN, an n-cladding layer 104 composed of n-AlnGa1-aN, a multi quantum well active layer (hereinafter referred to as an MQW active layer) 105, and a p-first cladding layer 106a composed of p-AlbGa1-bN are formed in this order on a sapphire substrate 102.
The MQW active layer 105 has a multi quantum well layer constructed by alternately stacking a plurality of quantum well layers composed of InxGa1-xN and a plurality of quantum barrier layers composed of InyGa1-yN, where x>y.
An n-current blocking layer 107 composed of n-AlcGa1-cN having a striped opening 108 is formed on the p-first cladding layer 106a. A p-second cladding layer 106b composed of p-AldGa1-dN and a p-contact layer 109 composed of p-GaN are formed in this order on the n-current blocking layer 107 and on the p-first cladding layer 106a inside the striped opening 108. A dotted line drawn in the striped opening 108 indicates the boundary between the p-first cladding layer 106a and the p-second cladding layer 106b. Here, 0≦a<c, 0≦b<c, and 0≦d<c.
A partial region from the p-contact layer 109 to the n-contact layer 103 is etched away, so that a surface of the n-contact layer 103 is exposed. A p electrode 110 is formed on the p-contact layer 109, and an n electrode 111 is formed on the exposed surface of the n-contact layer 103.
FIG. 23 is a schematic sectional view showing another example of the construction of a conventional GaN based semiconductor laser device having a current blocking layer.
In a semiconductor laser device 201 shown in FIG. 23, an n-contact layer 203 composed of n-GaN, an n-cladding layer 204 composed of n-AleGa1-eN, an MQW active layer 205, and a p-first cladding layer 206a composed of p-AlfGa1-fN are formed in this order on a sapphire substrate 202.
The MQW active layer 205 has a multi quantum well structure constructed by alternately stacking a plurality of quantum well layers composed of InaGa1-aN and a plurality of barrier layers composed of IntGa1-tN, where s>t.
A p-second cladding layer 206b in a ridge shape composed of p-AlfGa1-fN is formed on the p-first cladding layer 206a. An n-current blocking layer 207 composed of n-AlgGa1-gN having a striped opening 208 is formed on the p-first cladding layer 206a on both sides of the p-second cladding layer 206b. A p-contact layer 209 composed of p-GaN is formed on the n-current blocking layer 207 and on the p-second cladding layer 206b inside the striped opening 208. A dotted line drawn in the striped opening 208 indicates the boundary between the p-first cladding layer 206a and the p-second cladding layer 206b. Here, 0≦e<g and 0≦f<g.
A partial region from the p-contact layer 209 to the n-contact layer 203 is etched away, so that a surface of the n-contact layer 203 is exposed. A p electrode 210 is formed on the p-contact layer 209, and an n electrode 211 is formed on the exposed surface of the n-contact layer 203.
In the semiconductor laser devices 101 and 201, the Al composition ratios of the n-current blocking layers 107 and 207 are respectively higher than the Al composition ratios of the p-cladding layers 106a and 106b and the p-cladding layers 206a and 206b. Accordingly, the refractive indexes of the n-current blocking layers 107 and 207 are respectively lower than the refractive indexes of the p-cladding layers 106a and 106b and the p-cladding layers 206a and 206b. Consequently, effective refractive indexes in regions of the MQW active layers 105 and 205 under the striped openings 108 and 208 are respectively higher than effective refractive indexes in regions of the MQW active layers 105 and 205 under the n-current blocking layers 107 and 207. Accordingly, light is concentrated on the regions under the striped openings 108 and 208. A semiconductor laser device having a real refractive index guided structure is thus realized.
The semiconductor laser devices 101 and 201 shown in FIGS. 22 and 23 can have a loss guided structure by respectively composing the n-current blocking layers 107 and 207 of InGaN having a smaller band-gap than those of the active layers.
In the conventional semiconductor laser device 101 shown in FIG. 22, the n-current blocking layer 107 has the striped opening 108 which is rectangular in cross section. The width W of the striped opening 108 is approximately constant irrespective of the depth thereof.
In the conventional semiconductor laser device 201 shown in FIG. 23, the n-current blocking layer 207 has the striped opening 208 which is trapezoidal in cross section. The width of the striped opening 208 gradually decreases as the depth thereof decreases, that is, the lower width W2 is larger than the upper width W1.
In the semiconductor laser device 101 shown in FIG. 22, if the width W of the striped opening 108 is increased, an area occupied by the striped opening 108 in a plane shape of the semiconductor laser device 101 is increased. Even if the same operating voltage is applied to the semiconductor laser device 101, a current flowing into the MQW active layer 105 from the p-contact layer 109 through the striped opening 108 is increased. If the same light output power is achieved, the operating voltage can be decreased.
If the width W of the striped opening 108 is increased, however, the width of a light emitting spot in a direction parallel to the MQW active layer 105 is increased. Accordingly, the aspect ratio of laser light emitted from the semiconductor laser device 101 (a vertical divergence/horizontal divergence of emitted laser light) is increased.
Conversely, if the width W of the striped opening 108 is decreased, the width of the light emitting spot in the direction parallel to the MQW active layer 105 is decreased. Accordingly, the aspect ratio of the emitted laser light is decreased. However, a current flowing into the MQW active layer 105 from the p-contact layer 109 through the striped opening 108 is decreased. Accordingly, the operating voltage must be increased in order to make light output power constant.
Similarly in the semiconductor laser device 201 shown in FIG. 23, when the widths W1 and W2 of the striped opening 208 are increased, an operating voltage for obtaining the same light output power can be decreased, while the aspect ratio of emitted laser light is increased. Conversely, if the widths W1 and W2 of the striped opening 208 are decreased, the aspect ratio of the emitted laser light can be decreased, while the operating voltage is increased.
On the other hand, the realization of a semiconductor laser device in which an operating voltage is low and low-noise characteristics is obtained depending on the use has been desired.